The present invention relates to an optical recording medium, an optical recording method and an optical recording device.
As next-generation high-capacity optical disc systems, DVDxe2x88x92R and DVD+R are under development. In order to increase the recording capacity of DVDxe2x88x92R and DVD+R, it is necessary to develop recording materials that can reduce the size of a recording pit, to use image compression technique such as MPEG2 (motion picture experts group) and to develop semiconductor laser capable of emitting light having a short wavelength suitable for reading small recorded pits.
As for the laser diodes emitting light in the red region, AlGaInP semiconductor laser, which emits light having a wavelength of 670 nm, has been developed for commercial operation in bar code readers and measuring instruments. With an increase of the capacity of optical discs, red color semiconductor laser has been used in the optical data storage field. In the case of DVD drives, the standardized light source is light within the 630-690 nm band emitted by semiconductor laser. As for the read-only DVD-ROM (digital versatile disc-read-only-memory), driving systems using light having a wavelength of about 650 nm are marketed.
In the circumstances, DVDxe2x88x92R and/or DVD+R media capable of recording and reading data with a laser beam having a wavelength of 630-690 nm are desired. Various organic dyes have been hitherto proposed for use in a recording layer of an optical recording medium. The known media are designed such that the wavelength corresponding to the longer side end region of an absorption band in an absorption spectrum of a dye film, as shown in FIG. 1, is utilized for recording and reading information so as to obtain a high reflectance, which is one of the important characteristics of an optical recording medium using organic dyes. In this case, however, there arises a problem that the medium is greatly influenced by the wavelength of the laser beam.
It is known that the oscillation wavelength of a semiconductor laser for use in DVDxe2x88x92R and/or DVD+R varies depending upon the environment. In particular, at a high temperature, the oscillation wavelength shifts toward a longer wavelength side and, therefore, the absorbance coefficient of the dye used in the recording layer decreases, causing a reduction of the recording sensitivity thereof.
It is an object of the present invention to provide an optical recording medium suited for DVDxe2x88x92R and/or DVD+R system using a semiconductor laser having an oscillation wavelength in a short wavelength region.
Another object of the present invention is to provide an optical recording medium with reduced dependency on wavelength.
It is a further object of the present invention to provide an optical recording method and an optical recording device each using the above-described optical recording medium.
In accomplishing the foregoing objects, there is provided in accordance with the present invention an optical recording medium comprising a substrate, and a recording layer provided on said substrate wherein said recording layer comprises:
(a) a formazan metal chelate which comprises a formazan compound and a metal component,
(b) a squarylium metal chelate which comprises a squarylium compound and a metal component; and
(c) at least one additional dye selected from the group consisting of a phthalocyanine compound and a pentamethine cyanine compound.
In another aspect, the present invention provides an optical recording medium comprising a substrate, and a recording layer provided on said substrate wherein said recording layer comprises:
(a) a first formazan metal chelate which comprises a first formazan compound and a first metal component and which in the form of a film has the maximum absorption wavelength in the range of 500 to 650 nm,
(b) a squarylium metal chelate which comprises a squarylium compound and a metal component and which in the form of a film has the maximum absorption wavelength in the range of 500 to 650 nm; and
(c) a second formazan metal chelate which comprises a second formazan compound and a second metal component and which in the form of a film has the maximum absorption wavelength in the range of 650 to 750 nm.
The present invention also provides an optical recording medium comprising a substrate, and a recording layer provided on said substrate wherein said recording layer comprises:
(a) a first formazan metal chelate which comprises a first formazan compound and a first metal component and which in the form of a film has the maximum absorption wavelength in the range of 500 to 650 nm,
(b) a squarylium metal chelate which comprises a squarylium compound and a metal component and which in the form of a film has the maximum absorption wavelength in the range of 500 to 650 nm; and
(c) at least one additional dye which in the form of a film has the maximum absorption wavelength greater than 650 nm but not greater than 750 nm and which is selected from the group consisting of a phthalocyanine compound, a pentamethine cyanine compound and a second formazan metal chelate which comprises a second formazan compound and a second metal component.
The present invention also provides an optical recording method comprising the step of exposing the above optical recording medium to a light beam having a wavelength of 600 to 720 nm.
The present invention further provides an optical recording device comprising the above optical recording medium.